Field of the Invention
The present invention relates to stereoselective hydrogenation and in particular to the diastereoselective hydrogenation of bicyclic alkenes using known and novel hydrogenation catalysts.
Description of the Related Art
Heterogeneous hydrogenation of alkenes is a well-established technique. The mechanism of action of a metal catalyst supported on an inert solid is generally considered to occur through the absorption of hydrogen onto the catalyst metal surface and the subsequent approach of the carbon-carbon double band to that surface giving rise to hydrogen addition across one side of the double band. The planar structure of the alkene is transformed into the well-known tetragonal carbon structure, which may give rise to a particular set of chiral centre(s) depending upon the structure of the parent alkene. Metals used as catalyst are typically nickel, palladium, platinum or other precious metals (platinum group metals). Further review of the art can be found in Erti G., Knoezinger H., Schueth F., Weitkamp J.—Handbook of Heterogeneous Catalysis, John Wiley & Sons Inc., 2008, ISBN: 978-3-527-31241-2. Should the parent alkene be prochiral, hydrogenation in the presence of a heterogeneous metal catalyst is expected to give rise to a racemic mixture of enantiomers. Asymmetric heterogeneous hydrogenation, i.e. providing stereo isomeric products in unequal amounts, using a solid metal catalyst, has not have found the same industrial development as its asymmetric homogeneous hydrogenation counterpart and other strategies such as the use of a chiral modifier (e.g. tartaric acid or cinchonidine derivatives) adsorbed on the metal surface or covalently linked to the solid support of the metal catalyst as described in Murzin D. Y. et all, Catalysis Reviews: Science and Engineering, 2005, 47:2, 175-256, or in Ding K., Uozumi Y.—Handbook of Asymmetric Heterogeneous Catalysis, John Wiley & Sons Inc., 2008, ISBN 978-3-527-31913-8 are preferred as the logical route to influence chiral reaction pathways.
In the case of an unsaturated substrate bearing an existing chirality, the newly formed chiral centre will generate a mixture of diastereoisomers. In the absence of thermodynamic or kinetic controls, or other structural influencing effects, the mixture of diastereoisomers is expected to be a 50/50 mixture.
In some cases a functional group on an unsaturated (i.e. alkene) substrate bearing, or not, existing chirality, will have an asymmetric inductive effect, resulting in the preferential formation of one diastereoisomer aver the another. In Chemical Reviews, 1993, Vol. 93, 1307-1370, Hoveyda et al. describe several examples of directed heterogeneous hydrogenations where a functional group interacts with the metal surface of the catalyst, favouring the approach of the substrate to the catalyst by a specific side, and leading to the delivery of hydrogen to the unsaturation site in a syn fashion (with respect to the directing group). That disclosure provides that “the nature of the directing group, solvent, catalyst, support, and hydrogen pressure” influence the product. In the same paper, hydrogenations with homogeneous catalysts are also discussed, mostly on allylic or homoallylic substrates (cyclic or linear olefins). This method is based on the binding of hydrogen, the alkene unsaturation and the directing group to the metal centre of the catalyst, which is dependent on the electronic structure and configuration of the substrate, the metal and its ligands. Trial and error ‘fine tuning’ of the directing group (when possible), the catalyst and the reaction conditions is required to provide a good asymmetric induction through experiment. The high design flexibility of homogeneous metal catalysts allows for such fine tuning, but the end result can still prove expensive both in terms of catalyst and process costs (time, temperature, suitability for large scale use) when compared with standard commercial metal catalysts supported on an inert solid.
In Chemical Reviews 1999, 99, 1191-1223, A, Mengel et al. review the use in synthesis of reactions on olefins or carbonyls where the diastereoselectivity is induced by a remote stereocentre, and the different models used to predict their stereochemical outcomes, such as Cram's and Felkin-Anh's rules. Reactions involving an a-chiral double band, i.e. 1,2-induction, are the most common and cover a broad range of chemistry such as nucleophilic additions, electrophilic additions, cycloadditions or radical reactions. Moving the chiral centre to the position of the reaction centre, i.e. 1,3-induction, often requires the use of a chelating agent. In that case, either the reaction centre and the chiral centre are tethered together and the reagent is delivered externally of the chelate or the reagent becomes part of the chelate itself. Most reactions described are limited to nucleophilic additions, including carbonyl reductions with a metal hydride where the carbonyl and the chiral centre with an alcohol or ether functional group are first complexed with a Lewis acid.